Design And Synthesis Of Layered Double Hydroxides Based Heterostructure Materials Towards Energy-related Catalysis Explorations

With the increasing demand for energies,the current energy and chemical industry face lots of major challenges,such as resources shortage and environment pollution.The application of clean energy provides an effective pathway to achieve the sustainable development.Among various approaches to develop new clean energy sources,photocatalysis and electrocatalysis process have attracted tremendous attentions,in view of their cleanness,high efficiency and recyclability.The preparation of high performance catalysts is the key point to realize efficient photocatalysis and electrocatalysis process.For photocatalysis,the traditional semiconductor catalysts generally suffer from disadvantages of narrow responsive spectrum range and severe light corrosion.As for electrocatalysis,it meets the limitation of low electrical conductivity and agglomeration of active sites.Layered double hydroxides(LDHs)materials have shown great potential in the field of photocatalysis and electrocatalysis,due to its unique layered structure,tunability in both metal species and interlayer anions,and highly dispersed active sites.In this dissertation,structural design and controllable preparation of two kinds of energy-related catalysts(Cu2O@ZnCr-LDHs heterostructure and atomic layered Co/Co3O4)have been performed,which were used in photocatalytic overall water splitting and electroreduction of carbon dioxide,respectively.By virtue of regulation over band-gap structure of semiconductors and the construction of heterostructure,the available spectral range has been broadened and the stability has been improved.Based on a further precise regulation over the interfacial characteristics,two kinds of photo-and electro-catalysts with largely enhanced performance were obtained.The detailed results are presented as follows:1.Preparation of Cu2O@ZnCr-LDHs core-shell hollow nanospheres toward photocatalytic overall water splittingBy using Cu2O nanocubes as a soft template,vertical oriented ZnCr-LDHs array was in-situ fabricated on the surface of Cu2O to obtain Cu2O@ZnCr-LDHs hollow nanospheres.SEM and HRTEM observations reveal the well-crystallized hollow core-shell structure,with Cu2O nanocube core and ZnCr-LDHs shell size of?180 nm and?200 nm,respectively.The formation mechanism of this unique structure can be explained by the hard-soft-acid-base theory and Kirkendall effect via the coordination effect between Cu+ and S2O32-ions.Firstly,the dissolved Cu+ ion as a soft acid reacts with the soft base S2O32-,forming soluble[Cu2(S2O32-)n]2-2n and OH-.Subsequently,a co-precipitation reaction of Zn2+ and Cr3+ occurs under this alkaline condition to produce ZnCr-LDHs nanoplatelets on the surface of cubic Cu2O,accompanied with the inward etching of Cu2O caused by the Kirkendall effect.During the crystal growth of ZnCr-LDHs nanoplatelets,S2O32-anions enter into its gallery region to maintain the charge balance.This whole process results in the formation of hollow Cu2O@ZnCr-LDHs nanospheres with a thin-layer Cu2O core and S2O32-intercalated ZnCr-LDHs shell.In the further study of photocatalysis under visible light without any co-catalyst,the Cu2O@ZnCr-LDHs exhibits a high activity with H2 and O2 production rate of 0.90 and 0.44 ?mol h-1,respectively.Moreover,after 5 cycles and 20 h photocatalytic reaction,no obvious decrease in activity is observed,indicative of its excellent photostability.2.Modulation on the heterointerface of Cu2O@ZnCr-LDHs toward largely enhanced photocatalytic performanceTo deeply clarify the transfer mechanism of photo-induced electron-hole pairs at the heterointerface,synchrotron radiation combined with the molecular dynamics simulation were employed to prove the presence of Cu-(S2O32-)/LDH bridge structure in the Cu2O@ZnCr-LDHs heterostructure.A series of Cu2O@ZnCr-LDHs photocatalysts with tunable interfacial properties were prepared,by a precise regulation over Cu-(S2O32-)/LDH bridge structure.The Cu20O@ZnCr-LDHs photocatalyst with optimized interface exhibits an extremely high generation rate of H2 and 02 with 3.42 and 1.63 ?mol h-1,respectively,without any sacrificial agents or co-catalysts.This activity is among the highest level of reported photocatalysts under the same conditions.By using extended X-ray absorption fine structure(EXAFS)and coincidence Doppler broadened positron annihilation spectroscopy(CDB-PAS),for the first time,we substantiated that the Cu-(S2O32-)/LDH bridge structure extremely facilitates the interfacial transportation of photo-induced carrier via a Z-scheme route.In such a Z-schemed photocatalyst,upon light illustration,the photoexcited electrons from CB of ZnCr-LDH can inject into the VB of Cu2O through the bridge-type interface.The injected electrons neutralize the photogenerated holes of Cu2O and inhibit its self-oxidization,giving rise to a largely enhanced photostability.Different from the previous speculations and theoretical calculations,this work provides a direct experimental evidence for promoting the migration of carriers and improving the photocatalytic performance by the optimization of heterostructure interface.3.Preparation of atomic layered Co/Co3O4 toward carbon dioxide electroreductionSingle layer CoAl-LDHs nanosheet was obtained by delamination of bulk LDHs in formamide,which was subsequently used as substrate to growth ZIF-67 based on the complexation between cobalt atoms and 2-Methylimidazole.After the following calcination under oxygen-deficient conditions,partial Co atom in the LDH/ZIF-67 nanosheets was reduced,resulting in the formation of atomic layered Co/Co3O4.The delamination process makes a well-dispersed Co atoms in two-dimension LDHs laminate with a high exposure;and the presence of ZIF-67 inhibits the stacking aggregation of LDHs nanosheets during calcination.Most significantly,the carbonized framework derived from 2-MeIM ligands activates the cobalt atoms,giving rise to unsaturated and electron-rich cobalt active sites.This synergistic effect leads to a rather high electrocatalytic activity toward carbon dioxide electroreduction.The atomic layered Co/Co3O4 catalyst derived from completely delaminated LDHs shows a high current density of-8.27 mA cm-2 at-0.90 V(vs.SCE)with-300 mV overpotential.The current density is 30.70 and 4.38 times larger than that of the bulk(-0.269 mA cm-2)and partial delaminated(-1.89 mA cm-2)Co/Co3O4 catalysts,respectively.Moreover,the atomic layered Co/Co3O4 displays a Faraday efficiency over 80%toward formic acid(HCOOH)product,which can be potentially used in C1 and value-added chemical industry.